The present invention relates to a differential gearing unit for a vehicle, and in particular, to such a differential gearing unit including a torque responsive differential limiting function.
A differential gearing unit for a vehicle comprising an internal gear having internal teeth formed around an inner peripheral surface thereof, a sun gear disposed inside the internal gear concentrically therewith, a plurality of planet gears disposed between the internal gear and the sun gear in meshing engagement with both gears, and a planetary carrier for carrying the planet gears for revolution around the axis of rotation of the internal gear and the sun gear while allowing each of the planet gears to revolve on its own axis and constructed such that the planetary carrier is connected to a shaft which transmits a drive from an engine while the internal gear and the sun gear are connected to front and rear wheels of a four-wheel drive vehicle is known in the art.
FIG. 5 shows an example of a differential gearing unit for a vehicle as mentioned above, which is disclosed in Japanese Patent Application No. 2001-244,146 filed by the present applicant. Describing the construction of this prior art differential gearing unit with reference to FIG. 5, a housing 102 (102A, 102B) contains an internal gear 104 and a sun gear 106 disposed concentrically inside the internal gear 104. A plurality of planet gears 108 carried by a planetary carrier 110 are disposed between the internal gear 104 and the sun gear 106 in meshing engagement with both gears 104 and 106. In this example, all of the internal gear 104, the sun gear 106 and the planet gears 108 have twisted gear teeth which are in meshing engagement with each other. The planetary carrier 110 is secured to the housing 102 by bolts 112 for integral rotation.
The internal gear 104 includes a cylindrical portion 104e and a flange 104f which extends radially inward from one end (right end as viewed in FIG. 5) of the cylindrical portion 104e. Internal gear teeth 104b are formed on the inner peripheral surface of the cylindrical portion 104e while a spline groove 104a is formed in the inner peripheral surface of the flange 104f. A spline groove 122a formed around the outer periphery of an output member 122 fits in the spline groove 104a of the flange 104f. 
In the differential gearing unit for a vehicle as mentioned above, a spline groove formed in a shaft which transmits a drive from an engine may be engaged with a spline groove 110a formed around the inner periphery of the planetary carrier 110, for example, and one of front and rear wheels of a four-wheel drive vehicle may be connected to a spline groove 122b formed around the inner periphery of the output member 122 which is splined to the internal gear 104 while the other wheels may be connected to a spline groove 106b formed around the inner periphery of the sun gear 106.
With the differential gearing unit for a vehicle mentioned above, when a drive from the engine is transmitted and the vehicle is running straightforward on an even road, the carrier 110 which carries the planet gear 108 is driven for rotation, and there is no relative rotation between the planet gears 108 which are carried by the carrier 115 on the one hand and the internal gear 104 and the sun gear 106 which mesh therewith on the other hand, resulting in an integral rotation. When the vehicle is turning around a corner of the road, the revolution of the planet gears 108 carried by the carrier 110 on their own axes provides for a differential rotation between the front and the rear drive wheels which are connected to the internal gear 104 and the sun gear 106, respectively. When there is a difference in the coefficient of friction which the front and the rear wheels experience from the road surface to produce a difference in the grip between the front and the rear drive wheels, an axial thrust which results from a reaction to the rotation of the meshing twisted gear teeth and frictional forces which are developed at the abutting surfaces between the carrier 110 and the planet gears 108 provide a torque distribution or a differential limiting function.
A conventional internal gear 104 which is assembled into the differential gearing unit for a vehicle as mentioned above has internal gear teeth 104b which are formed by a cutting operation. This requires the provision of a work grinding undercut when machining an innermost portion (a portion indicated by character A in FIG. 5) of the internal gear teeth 104b, presenting a problem that the axial length is increased and another problem that a mechanical strength may become wanting because of a reduced thickness of this portion.